It has long been known that combustion efficiency and exhaust emissions can be improved by injecting a small amount of fuel into the combustion chamber before main injection begins. This pre-injection is oftentimes referred to in the art as pilot injection and/or rate shaping. In the field of hydraulically actuated fuel injectors, pilot injection can be accomplished in a number of ways. One method is by controlling the initial velocity profile of a plunger that pressurizes the fuel at the beginning of each injection cycle. The movement of the plunger in a hydraulically actuated fuel injector can in turn be controlled by controlling the flow rate of the high pressure hydraulic fluid acting on the top face of the piston that supplies the downward force to the plunger. Thus, pilot injection can be accomplished by controlling the initial flow rate of the high pressure hydraulic fluid acting on the top surface of the piston in such a way that the piston hesitates momentarily in its downward movement.
One known method for creating an initial hesitation in the piston is to design geometrical relationships between the piston and the piston bore that prevent the high pressure hydraulic fluid from acting over the complete surface of the piston when the piston begins its downward movement. In other words, by exposing only a portion of the piston to high pressure hydraulic fluid initially, the piston hesitates in its downward movement until the complete upper surface of the piston is exposed to the high pressure hydraulic fluid. Unfortunately, these prior art geometrical interrelationships require such a high degree of precision machining that mass production of these injector components was not economically realistic. For instance, U.S. Pat. No. 3,921,604 to Links describes a fuel injector having an intensifier piston with a conical protuberance on its top side that projects into the high pressure hydraulic fluid supply bore. Links describes this geometry as giving the injector the ability to control the stroke speed of the piston, presumably because the conical portion prevents the high pressure fluid from flowing quickly to act on the remaining surface area of the piston. While Links does recognize that some injection rate shaping capability can be accomplished by the geometrical interrelationship between the piston and the high pressure hydraulic fluid supply bore, the Links geometry suffers from a number of disadvantages which render it difficult to reliably predict performance due to extreme sensitivity to machining tolerances.
In Links, there are several features of the piston and bore that have a significant influence on the velocity profile of the piston, which in turn controls the ejection rate profile. Among these features are bore diameter, base diameter of the conical protuberance, the height of the protuberance, the perpendicularlity of the piston shoulder surface and the perpendicularlity of the bore shoulder seating surface. Since all of these geometrical features of Links must be held to extremely tight tolerances, the Links injector is difficult to produce in large quantities with reliable and predictable performance.
The present invention is directed to overcoming one or more of the problems as set forth above.